This invention concerns boat lifting devices, in particular one designed to lift a boat or other vessel along a single, rigid H-beam using a bi-support roller system having wheels contacting the web and inside flange surfaces of the H-beam.
It has long been appreciated that most boats will last longer if protected from the elements when not in use. To help protect the boats, boathouses have often built adjacent piers. The boathouse not only protects the boat from the rain and the seas, but they are often also outfitted with hoists to lift the boat out of the water when not in use. In addition to keeping the boat out of the water, boathouses also permit easy access to and from the boat. This is especially important in places where the water level rises seasonably, as in many fresh water lakes, or daily, as in tide influenced areas. Further, when the boat is on a hoist, it is often much more stable than when floating to further aid getting on and off the boat.
Although boathouses with overhead hoists have many advantages, in some areas, such as Lake Tahoe between California and Nevada, there are zoning restrictions which prohibit one from erecting a boathouse. One way to get around this restriction is by the use of a boat lift, which lifts the boat from below, rather than a hoist which lifts a boat from above.
Boat lifts must accommodate vertical forces from the weight of the boat, side-to-side, lateral twisting forces caused by waves and wind tending to move the boat, and vertical twisting forces caused by the boat not being exactly centered on the under-hull support members. Because of these forces, many of the prior art boat lifts use a number of submerged vertical posts and other support members to help reduce the problems associated with accommodating these large vertical and twisting forces. See U.S. Pat. No. 4,401,325 (Godberson).
Another type of boat lift, which looks like a fork lift with a single support driven into the bottom of the body of water, have been used at private piers at Lake Tahoe, Calif. for many years. An early version uses an H-beam as the main support with bearings mounted to the external surfaces of the H-beam flanges. This was found to cause binding if the boat was not exactly centered on the fork lift style tines. Therefore, later versions used a square tube as the main support. A number of rollers or other bearing members, mounted to a roller carrier, engage the outer surface of the square pipe. See U.S. Pat. No. 4,482,268 for an example of a more recent version of a single support type of boat lift using a square tube main support.
In many parts of the country, water levels change dramatically even in the course of 24 hours. This is especially true during ebb tide when the pier is much higher than the water level. Some of the prior art boat lifts have under-hull support projections below the frames. These projections can prevent the frame from being lowered as far as needed when the waters are extremely shallow. See, for example, U.S. Pat. No. 4,482,268 (Stephenson, et al.), U.S. Pat. No. 3,753,355 (Knoch), and U.S. Pat. No. 3,362,172 (Rutter). Also, with some prior art boat lifts, the boat cannot be lifted above the pier level, presenting problems during flow tides or other unusually high water levels. See, for example, U.S. Pat. No. 4,432,664 (Baldyga).